Analysis of Development Methods for Gravel Envelope Wells - 13
3.3 Rocker Beam Swabbing
As noted in Section 2.3, peak velocities obtained are, for the same
head across the swab, equal to those for line swabbing. However, the effect of the
valve in the swab, which allows it to fall back, results in a reduction in the head
available to produce a flow through the filter pack. The effect of the rocker beam
motion is to "pulsate" the flow in the filter pack. Although head difference available
may be reduced by valve action, accelerations associated with the oscillatory motion
would provide an excess difference equal to a modification of the gravitational
acceleration. A 3 ft stroke and period of 2 seconds results in a peak acceleration of
1.5 
2 =
14.8 ft/ sec2, or about g/2. Thus peak tangential velocity through the
filter pack is increased by about 50%.
As with the line swabbing the radial effect can be split into a
component representing production and a component representing swab bypass flow through
the filter pack. Radial flow induced by the production component is governed by the
depth of well screen below the swab and peak swab velocity. Peak swab velocity for a 3
ft stroke and period of 2 seconds is 1.5 ft/sec or about 4.7 ft/sec. Given equal heads, radial
velocities are about 50% greater than for line swabbing. Flows are repeated every two
seconds so there will be steady migration of particles through the well screen.
Given the pulsating character of flow fields produced by rocker beam
swabbing it is probably an effective development tool both for flushing of drilling
debris and wall cake and consolidation of the filter pack.
As stated in Section 2.4, peak tangential velocity at the formation /
filter pack interface for a stationary swab is equal to three times the scaling velocity
v* defined by Hk1/a. The pumping head provides bypass flow, while increase in static
head develops a recharge flow. Allowance must be made for the pumping head used to
overcome pipe friction.
Suppose a tangential velocity of 1.5 inches/second is considered
adequate at the filter pack/formation interface. For a 14-inch diameter well and 7-inch
filter pack this is accomplished with a head difference of 10 feet across the swab.
Flow that must be delivered to attain this head difference depends on total screen
length. Since the entire open well screen is available for recharge, flow to be
delivered is controlled by total screen length and formation recharge capability. In
any situation the operator can easily ascertain what is occurring from observation of
pumping pressure and volume of makeup water required.
Swab motion produced by hauling and dropping the drill pipe creates
significant velocities in the filter pack. A typical fall velocity is about 8 ft/ sec.
With 14-inch diameter well screen this produces a flow of 3800 gpm, which is generally
in excess of injection pumping capacity. Without swab clearance this flow must either
be forced into the formation or bypass the swab through the screen. Such flows forced
into the filter pack will almost certainly fluidize it and be very effective in scouring
wall cake from the borehole. However, if there is considerable screen below the swab,
some flow could go into the formation as temporary recharge.
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